Electrochemical CO₂ reduction to C₂₊ products over Cu/Zn intermetallic catalysts synthesized by electrodeposition

Abstract

Electrocatalytic CO₂ reduction (ECR) offers an attractive approach to realizing carbon neutrality and producing valuable chemicals and fuels using CO₂ as the feedstock. However, the lack of cost-effective electrocatalysts with better performances has seriously hindered its application. Herein, a one-step co-electrodeposition method was used to introduce Zn, a metal with weak *CO binding energy, into Cu to form Cu/Zn intermetallic catalysts (Cu/Zn IMCs). It was shown that, using an H-cell, the high Faradaic efficiency of C₂₊ hydrocarbons/alcohols ({\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}}) could be achieved in ECR by adjusting the surface metal components and the applied potential. In suitable conditions, FE_C2+ and current density could be as high as 75% and 40 mA/cm², respectively. Compared with the Cu catalyst, the Cu/Zn IMCs have a lower interfacial charge transfer resistance and a larger electrochemically active surface area (ECSA), which accelerate the reaction. Moreover, the *CO formed on Zn sites can move to Cu sites due to its weak binding with *CO, and thus enhance the C–C coupling on the Cu surface to form C₂₊ products.

Graphical abstract

Keywords

carbon dioxide electroreduction / electrochemistry / co-electrodeposition / intermetallic catalysts / value-added chemicals

1 Introduction

In the context of global warming and the energy crisis, the search for novel, sustainable, and efficient ways to produce energy and chemicals has become a hot issue of common concern for scientists worldwide. Therefore, transforming CO₂ into useful chemicals and fuels has become an important method for mitigating climate change and alleviating the energy crisis [1–4]. The use of clean and renewable energy to perform electrocatalytic CO₂ reduction (ECR) to generate carbon-containing chemicals will hopefully address global warming and the energy crisis [5–8]. Among the metallic catalysts applied in ECR, Cu has been considered as the most promising metallic catalyst for ECR industrial applications, because it combines favorably with most intermediates of ECR to produce a variety of value-added multi-carbon products. However, the selectivity of Cu for specific products and stability remains poor in ECR, which seriously increases the difficulty and cost of product purification in practical industrial applications [9,10]. Therefore, in recent years, researchers have begun to explore the use of novel electrocatalysts to enhance the stability, efficiency, and selectivity of ECR.

Intermetallic catalysts (IMCs) possess a regular surface or near-surface atomic ordering structure and unique electronic properties, which have shown an excellent catalytic performance in many chemical reactions, attracting widespread attention [11]. Higher mixing enthalpies and stronger atomic interactions in IMCs catalysts compared to monometallic catalysts help to inhibit catalyst recombination, improving the activity and stability of ECR [12,13]. CuAu intermetallic catalysts were studied in ECR and gaseous products were produced over Cu-rich phases [14–17]. CuZn and CuPd intermetallic catalysts were also studied [18–21]. For example, Kuang et al. [22] developed a strategy to synthesize CuAu intermetallic catalysts with ordered atomic arrangements at low temperatures and applied it to ECR, and found that it could selectively reduce CO₂ to CO with a good stability. In another research, Juntrapirom et al. [19] synthesized a Cu/Zn-MOF catalyst by one-step carbonization supported on conductive porous carbon material and used it for the preparation of C₂ products (including C₂H₄ and CH₃CH₂OH) by ECR. However, conventional preparation methods are complex and difficult to achieve precise adjustment of the microstructure of IMCs catalysts, which hinders the establishment of intuitive and reliable conformational relationships and identification of active sites.

Herein, a one-step co-electrodeposition method to prepare Cu/Zn IMCs was reported. Using this as electrodes, a Faradaic efficiency of 75% of C₂₊ hydrocarbons/alcohols ({\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}}) and 40 mA/cm² of a total current density were obtained in ECR using an H-cell. The reason for Cu/Zn IMCs to promote C₂₊ products and enhance the current density compared with pure Cu was explored. This enhancement was caused by tandem catalytic and electronic effects of nanoscale Cu and Zn domains through the interfacial region.

2 Experimental section

2.1 Chemicals and materials

The copper (II) gluconate (C₁₂H₂₂CuO₁₄, 98%) was purchased from Strem Chemicals, Inc. The zinc acetate (C₄H₆O₄Zn, 99.99% metals basis), cesium iodide (CsI, 99.9%), sulfuric acid (H₂SO₄), dimethyl sulfoxide (DMSO, AR), phenol (AR), and potassium sulfate (K₂SO₄, 99%) were obtained from Meryer Co., Ltd. The carbon paper (CP, TGP-H-60) and the proton exchange membrane (N-117, 0.180 mm thick) were purchased from Suzhou Sinero Technology Co., Ltd. The Nafion^TM solution (1100W, 5 wt.%) was obtained from Sigma-Aldrich. The CO₂ and N₂ (Shanghai Air Liquide Gas Co., Ltd.) had a purity of 99.999% and were used as received.

2.2 Catalyst preparation

2.2.1 Preparation of pure Cu or pure Zn electrodes

The self-supported pure Cu or pure Zn electrodes were deposited on a piece of hydrophobic carbon paper (CP, 1.1 cm²) [23]. The electrolyte tank for deposition was a single chamber cell equipped with a platinum electrode as a counter electrode. For pure Cu or pure Zn electrodes, electrodeposition was performed in a mixed electrolyte of 50 mL aqueous solution with 0.01 mol/L of H₂SO₄ and 0.1 mol/L of C₁₂H₂₂CuO₁₄ or 0.01 mol/L of H₂SO₄ and 0.1 mol/L of C₄H₆O₄Zn. Electrostatic control was performed by a DC power supply (Hangzhou Huayi Electronic Industry Co., Ltd.). The pure Cu or pure Zn electrodes were electrodeposited at a deposition potential of 3 V for 2 min. The prepared electrodes were washed 5 times with ultrapure water and stored in a vacuum environment.

2.2.2 Preparation of Cu_xZn_y IMCs electrodes

The self-supported Cu₁₀₀Zn_4.9 IMCs electrodes were electrodeposited on a piece of hydrophobic CP. The electrolyte tank for deposition was a single chamber cell equipped with a platinum electrode as a counter electrode. For Cu₁₀₀Zn_4.9 IMCs electrodes, electrodeposition was performed in a mixed electrolyte of 50 mL of aqueous solution with 0.01 mol/L of H₂SO₄, 0.1 mol/L of C₁₂H₂₂CuO₁₄, and 0.05 mol/L of C₄H₆O₄Zn. Cu_xZn_y IMCs electrodes with different mole ratios were deposited at a deposition potential of 3 V for 2 min (Cu_xZn_y IMCs; x/y: mole ratio of Cu/Zn and the values of x and y were determined by the inductively coupled plasma optical emission spectroscopy (ICP-OES), which was shown in Table S1). The prepared electrodes were washed 5 times with ultrapure water and stored in a vacuum environment.

2.3 Catalyst characterization

The morphology of catalyst was characterized by a scanning electron microscope (SEM, Zeiss S450) equipped with energy dispersive X-ray spectroscopy (EDS) operated at 5 kV and a transmission electron microscope (TEM, JEM-2100F) operated at 200 kV. The powder X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer (Rigaku Ultima VI X-ray) using Cu-Kα radiation. The diffractograms were collected in the range of 5°–90° with a scan rate of 10° min⁻¹. The X-ray photoelectron spectroscopy (XPS) data were conducted on an AXIS Supra surface analysis instrument. C 1s (284.6 eV) was used as a reference, and the binding energy was corrected. The content of metals in the catalysts was determined by ICP-OES (Optima 8300, Perkin-Elmer). The X-ray adsorption spectroscopy (XAS) was measured at the 4B9A beamline at Beijing Synchrotron Radiation Facility.

2.4 Electrocatalytic measurements

All electrocatalytic measurements were conducted in an H-cell on the electrochemical workstation (CHI 660E). The prepared Cu_xZn_y IMCs electrode was used as cathodic working electrode. Ag/AgCl (saturated KCl solution) and Pt electrodes were used as reference electrode and counter electrode, respectively. 0.1 mol/L CsI and 0.1 mol/L K₂SO₄ solutions were used as cathode and anode electrolytes, respectively. The potential (for Ag/AgCl) was transformed to the reversible hydrogen electrode (RHE) by Eq. (1).

(1)\begin{array}{rl}E(\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{u}\mathrm{s}\mathrm{R}\mathrm{H}\mathrm{E})=& E(\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{u}\mathrm{s}\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l})\\ & +0.197\mathrm{V}+0.059\times \mathrm{p}\mathrm{H}.\end{array}

CO₂ or N₂ was continuously passed for a certain period to ensure that a gas-saturated electrolyte was formed before the experiment, prior to the experiment. Linear sweep voltammetry (LSV) measurements were performed over the potential range of 0 to −1.4 V versus RHE with a sweep rate of 20 mV/s.

The electrochemically active surface area (ECSA) of the electrodes was analyzed by the measurement of the double-layer capacitance (C_dl). The C_dl value was obtained by recording cyclic voltammetry (CV) at various scan rates in the region of non-faradaic in an H-cell. Electrochemical impedance spectroscopy (EIS) measurements were made in a CO₂-saturated 0.1 mol/L CsI solution with an open-circuit potential (OCP) frequency range of 0.01 Hz to 100 kHz and an applied voltage amplitude of 5 mV.

2.5 Product analysis

To explore the performance of ECR, the catalysts were tested by potentiostatic electrolysis for 30 min in an H-cell. A gas chromatography (GC, Agilent-7890A) with a TCD detector was used to analyze the products of gas produced by ECR. The liquid products generated by 30 min of potentiostatic reduction in an H-cell was analyzed by NMR spectrometer (Bruker; Ascend 400–400 MHz), and 1H NMR analysis was performed in 400 μL of electrolyte with 200 μL of D₂O, 100 μL of phenol (200 mmol/L), and 100 μL of DMSO. Phenol was an internal standard for formic and DMSO was an internal standard for ethanol, acetic, and isopropanol. The FE of the product was calculated using Eq. (2).

(2)FE=n\times F\times \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}/Q_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}\times 100\mathrm{\%},

where Q is the amount of charge, F is the constant of Faraday, and n is number of transferred electrons.

3 Results and discussion

3.1 Preparation and structural characterization of electrocatalysts

As displayed in Fig.1(a), Cu/Zn IMCs were prepared by a simple one-step co-electrodeposition method, which makes it easier to control the nucleation rate of metal nanocrystals and their morphological control (please refer to the experimental section for details). The IMCs were grown on CP fiber by the co-electrodeposition method which is shown in Fig.1(a), using Cu₁₀₀Zn_4.9 IMCs as a representative example (referred to as “Cu_xZn_y IMCs,” x/y: mole ratio of Cu/Zn, and the values of x and y were determined by the ICP-OES, which was shown in Table S1). Representative SEM images of pure Cu, pure Zn, and Cu₁₀₀Zn_4.9 IMCs are displayed in Fig.1(b)–Fig.1(d). As shown, the nanoparticle diameters of Cu₁₀₀Zn_4.9 IMCs are significantly smaller than those of pure Cu and pure Zn, suggesting that the introduction of metallic Zn can control the morphology of the catalysts, resulting in smaller nanoparticles of Cu₁₀₀Zn_4.9 IMCs and the particles are uniformly dispersed on the surface of the CP fibers, which helps expose more active sites [6,21]. TEM was used to directly observe the microstructure of electrocatalysts and the matching images are displayed in Fig.1(e) and S1. In the TEM image of the Cu₁₀₀Zn_4.9 IMCs catalyst, the lattice fringe measured was 0.236 nm. By comparison, the lattice fringes of Cu₁₀₀Zn_4.9 IMCs were different from pure Cu (0.209 nm) or pure Zn (0.231 nm) prepared by a similar one-step electrodeposition method, demonstrating successful preparation of the Cu/Zn IMCs in Fig. S2 [24–26]. During the preparation of Cu₁₀₀Zn_4.9 IMCs catalysts, Zn was inserted into the lattice of Cu under the conditions of electrodeposition, resulting in a uniform distribution of Cu and Zn elements throughout the Cu₁₀₀Zn_4.9 IMCs nanoparticles, as shown in the EDS image of Fig.1(f).

The prepared sample catalysts were analyzed by XRD. Fig.1(g) and S3 present XRD patterns of different electrodes. Fig.1(g) shows the XRD patterns of pure Cu, pure Zn, and Cu₁₀₀Zn_4.9 IMCs. The diffraction peaks of pure Cu and Zn are consistent with those reported in Refs. [27,28]. There were six peaks at 36.3°, 39.0°, 43.2°, and 70.6° in pure Zn corresponding to the (002), (100), (101), and (110) lattice planes of metallic Zn (PDF#04-0831) [27]. There were also three peaks at 43.3°, 50.4°, and 74.1° in pure Cu, which are features of metallic Cu (PDF#04-0836) [28]. Similar to the above results, the Cu content in Cu₁₀₀Zn_4.9 IMCs was dominant and the diffraction peaks of Zn were weaker than the diffraction peaks of Cu. The deposition reduction potentials of Cu and Zn are different during the electrodeposition process, and the XRD results show that Cu was deposited in preference to Zn in this experiment, as the aim of this work was to construct Cu sites capable of C–C coupling reactions.

The chemical composition and valence states of Cu₁₀₀Zn_4.9 IMCs were characterized by XPS of Cu 2p and Zn 2p patterns, as shown in Fig.2(a). In this study, the deconvolution of Cu 2p provides two major peaks at 932.52 and 952.22 eV that are considered as Cu⁺/Cu⁰ species. This implies that the Cu elements in Cu₁₀₀Zn_4.9 IMCs are mainly formed by Cu⁰ and Cu⁺ [29]. It is obviously observed that the Cu 2p peaks of Cu₁₀₀Zn_4.9 IMCs are stronger than pure Cu. In addition, a weak peak that appears in Cu 2p of Cu₁₀₀Zn_4.9 IMCs may be caused by the introduction of Zn. Comparing the above results, it can be known that the Cu₁₀₀Zn_4.9 IMCs have a lower content of Zn, despite the fact that Zn 2p peaks were measured in the Cu₁₀₀Zn_4.9 IMCs [30]. Subsequently, in order to elucidate the coordination structure of Cu in Cu₁₀₀Zn_4.9 IMCs, the Cu K-edge was analyzed using XAS (Fig.2(b)). For comparison, the reference spectra of the metals Cu foil, Cu₂O, and CuO were also provided. By observing the X-ray absorption near-edge structure (XANES) spectra of pure Cu and Cu₁₀₀Zn_4.9 IMCs, it is found that the difference between these materials is due to the introduction of Zn. This means that in Cu₁₀₀Zn_4.9 IMCs, there may be an electron transfer between Cu and Zn and the change in electronic properties may also affect their catalytic properties [29]. The results show that the valence of Cu in pure Cu and Cu₁₀₀Zn_4.9 IMCs is between 0 and 1 [31]. Fig.2(c) exhibits a representative set of Fourier-transformed (FT) extended X-ray absorption fine structure (EXAFS) spectra of pure Cu and Cu₁₀₀Zn_4.9 IMCs. The result shows a peak of about 2.2 Å for pure Cu and Cu₁₀₀Zn_4.9 IMCs, attributed to the Cu–Cu bond, which is consistent with the XANES results [32]. After the introduction of Zn, the peak Cu–Cu bond of Cu₁₀₀Zn_4.9 IMCs is slightly shifted, due to the formation of the Cu–Zn bond, which is also consistent with the XPS results. Another distinct peak of Cu₁₀₀Zn_4.9 IMCs at 1.6 Å can be associated with the Cu–O bond compared to the standard sample. Taking the wavelet transform (WT) spectra of the samples (Cu foil, Cu₂O, and CuO) as a reference (Fig.2(d) and S4), it is observed that pure Cu is nearly the same as Cu foil, while Cu₁₀₀Zn_4.9 IMCs are obviously different from Cu foil. Combining all the above results, it is concluded that the preparation of Cu/Zn IMCs is successful. The introduction of Zn changes electronic structure of Cu, which should be favorable to promoting reduction reactions due to the active electronic state [33,34].

3.2 Electrochemical performance test of catalysts

The ECR activity and durability of pure Cu and Cu₁₀₀Zn_4.9 IMCs were then evaluated with an H-cell in CO₂-saturated 0.1 mol/L of CsI solution, where water was used as the proton source, whose results were shown in Fig.3. The LSV at the electrodes of pure Cu and Cu₁₀₀Zn_4.9 IMCs in CO₂- or N₂-saturated 0.1 mol/L of CsI aqueous electrolytes are shown in Fig.3(a). The LSV and CV curves of Cu₁₀₀Zn_4.9 IMCs show a higher current density in the potential range and a low initial potential of the reaction compared to pure Cu (Fig. S5). The lower current density in N₂-saturated 0.1 mol/L of CsI solution implies the occurrence of ECR [35]. It is commonly accepted that the increase in electrocatalytic activity is usually associated with an increase in active sites [36]. Therefore, the ECSA was defined by measuring the C_dl in Fig.3(b). The ECSA of Cu₁₀₀Zn_4.9 IMCs was found to be significantly larger than that of pure Cu, indicating more active sites on Cu₁₀₀Zn_4.9 IMCs. This enhancement may be attributed to the fact that the introduction of Zn reduces the average particle size of the catalyst and the stronger electron transfer between Cu and Zn in Cu₁₀₀Zn_4.9 IMCs. Fig.3(c) displays the Nyquist plots of pure Cu and Cu₁₀₀Zn_4.9 IMCs tested with EIS to explore the reaction kinetics. The smaller semicircular diameter of the high-frequency droplets indicates less resistance to charge transfer and less evolution of the reaction kinetics [16]. Cu₁₀₀Zn_4.9 IMCs have a lower interfacial charge transfer resistance and therefore have a higher electrical conductivity, which enhances the charge transfer kinetics during ECR. The enhanced performance of ECR and charge transfer kinetics can be credited to the enhancement of conductivity and the change of electronic structure [37].

To explore the role of Zn introduction for the activity and selectivity in ECR, the current density and the product distribution over pure Cu and Cu₁₀₀Zn_4.9 IMCs were analyzed and the results are provided in Fig.3(d)–Fig.3(f). Fig.3(d) shows that the total and partial current density of C₂₊ over Cu₁₀₀Zn_4.9 IMCs is higher than that over pure Cu since Cu and Zn are in a tandem reactor process [38]. At −1.28 V versus RHE, the current density over Cu₁₀₀Zn_4.9 IMCs can approach 40 mA·cm⁻², and the partial current density of the C₂₊ product is about 30 mA·cm⁻². In contrast, the partial current density of C₂₊ over the pure Cu electrode is only 10 mA·cm⁻². When the potential is further increased, the partial current density of C₂₊ products does not increase obviously because of the decrease of {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}} products. As seen in Fig.3(d) and Fig.3(f), the overall FE of products from CO₂ on the pure Cu is lower than that on the Cu₁₀₀Zn_4.9 IMCs. The introduction of Zn into Cu suppresses the HER and the reduction products from CO₂ increase significantly. At −1.28 V versus RHE, the FE of the C₂₊ reaches 75%. In contrast, the {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}} at −1.28 V versus RHE is only 32.2% because of severe HER. Systematic comparisons to state-of-the-art catalysts revealed that the performance of the Cu₁₀₀Zn_4.9 IMCs was one of the best electrocatalysts in the H-type cell (Table S2).

3.3 Effect of Cu/Zn mole ratio on electrocatalytic performance

To further investigate the modulation of addition of Zn with the electrocatalytic performance, the mole ratio of Cu/Zn was adjusted. As shown in Fig.4(a) and Fig.4(b), the FE of the C₂₊ and the C₂₊/C₁ product ratio increases with increasing Zn content. The maximum ratio occurs at a Cu/Zn mole ratio of 100/4.9, showing an overall volcanic pattern. One of the main reasons for the enhancement of FE_C2+ by introducing a suitable amount of Zn into Cu may be that Zn has weak binding energy with *CO, and the *CO formed on Zn sites moves to the Cu sites and increases *CO coverage, which is favorable to C–C coupling on the Cu surface [39,40].

3.4 Electrocatalytic stability test

In addition to the catalytic activity, the electrolytic stability, especially operating at high current densities, is another key indicator of the practicality of CO₂ electrolysis technology [41,42]. Fig.5 demonstrates the stability of Cu₁₀₀Zn_4.9 IMCs at −1.28 V versus RHE. It shows that the current density and {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}} does not change obviously during 8 h of electrolysis, indicating the excellent stability of the catalyst [43,44]. After the reaction, XAS analysis was performed and the results showed that the properties of the catalyst did not change noticeably (Fig. S6). The EXAFS at the Cu K-edge confirmed a well-retained Cu interaction in samples collected after the ECR test. These results also indicated the remarkable stability of the Cu₁₀₀Zn_4.9 IMCs catalyst.

4 Conclusions

In conclusion, Cu/Zn IMCs electrodes can be prepared by one-step co-electrodeposition. The obtained IMCs exhibit an excellent ECR performance for producing C₂₊ products. In particular, the Cu₁₀₀Zn_4.9 IMCs yield 75% of FE_C2+ with a current density of 40 mA/cm² using an H-cell. Compared with pure Cu catalyst, Cu/Zn IMCs have a lower interfacial charge transfer resistance and a larger ECSA, which are favorable to the reaction. Moreover, the *CO formed on Zn sites can move to Cu sites due to its weak binding with *CO, and thus enhance the C–C coupling on the Cu surface to form C₂₊ products. It is believed that the electrodeposition method has a great potential for the design and synthesis of efficient electrocatalysts with intermetallic structures.

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